Epoxy resin compositions

ABSTRACT

In an epoxy resin composition comprising an alicyclic epoxy resin, a curing agent having phenolic hydroxyl group in its molecule and an imidazole curing accelerator, excellent thermal resistance and excellent mechanical properties can be given to the cured epoxy resin composition by blending 100 parts by weight of 2-(3,4-epoxy)cyclohexyl-5,5-spiro(3,4-epoxy)cyclohexane-m-dioxane as the alicyclic epoxy resin, 25 to 50 parts by weight of a phenol-formaldehyde condensate as the curing agent having phenolic hydroxyl group in its molecule and 0.1 to 15 parts by weight of at least one imidazole compound selected from the group consisting of 2-phenylimidazole, 2-undecylimidazole, 2-heptadecylimidazole, 2-phenyl-4-methylimidazole, 1-cyanoethyl-2-methylimidazole, 1-cyanoethyl-2-ethyl-4-methylimidazole, 1-cyanoethyl-2-methyl-4-ethylimidazole, 1-cyanoethyl-2-undecylimidazole, 1-cyanoethyl-2-phenylimidazole, 1-azine-2-methylimidazole, 1-azine-2-undecylimidazole and 1-azine-2-ethyl-4-methylimidazole as the imidazole curing accelerator.

BACKGROUND OF THE INVENTION

The present invention relates to an epoxy resin composition. Moreparticularly, the invention pertains to an epoxy resin compositioncomprising an alicyclic epoxy resin, a curing agent having phenolichydroxyl group in its molecule and an imidazole curing accelerator.

It is well known that epoxy resins are widely used as electricinsulating materials, for example, as an insulating material forconductors of rotary electric machines. There are provided various epoxyresin compositions, any one of which is far superior to the otherthermosetting resins such as phenol resin in properties such aselectrical properties and thermal resistance. However, the epoxy resincompositions are considerably inferior to silicone resins particularlyin thermal resistance, and are still unsatisfactory as an insulatingmaterial for large rotary electric machines which require excellentthermal resistance, for example, thermal resistance of at least class H(at least 180° C). Also, as a resin used in the prepreg insulationmethod which is recently often employed, the epoxy resin compositionsare unsatisfactory in the storage stability of resin compositions(varnishes), curability, flexibility after semicure, that is, whenformed into a prepreg tape, and thermal resistance (heat deteriorationcharacteristic) and mechanical strengths, particularly bending strength,after cure. Particularly, there is no epoxy resin composition which canbe used as a resin for prepreg insulation method having thermalresistance of at least class H.

An object of the present invention is to provide an epoxy resincomposition which can give a cured product having excellent thermalresistance and excellent mechanical properties.

Another object of the invention is to provide an epoxy resin compositionwhich shows good flexibility in a dry state or a semi-cured state.

SUMMARY OF THE INVENTION

According to the present invention, there is provided an epoxy resincomposition comprising an alicyclic epoxy resin, a curing agent having aphenolic hydroxyl group in its molecule and an imidazole curingaccelerator, characterized in that said composition is obtained byblending 100 parts by weight of2-(3,4-epoxy)cyclohexyl-5,5-spiro(3,4-epoxy)cyclohexane-m-dioxane as thealicyclic epoxy resin, 25 to 50 parts by weight of a phenol-formaldehydecondensate as the curing agent having phenolic hydroxyl groups in itsmolecule and 0.1 to 15 parts by weight of at least one imidazolecompound selected from the group consisting of 2-phenylimidazole,2-undecylimidazole, 2-heptadecylimidazole, 2-phenyl-4-methylimidazole,1-cyanoethyl-2-methylimidazole, 1-cyanoethyl-2-ethyl-4-methylimidazole,1-cyanoethyl-2-methyl-4-ethylimidazole, 1-cyanoethyl-2-undecylimidazole,1-cyanoethyl-2-phenylimidazole, 1-azine-2-methylimidazole,1-azine-2-undecylimidazole and 1-azine-2-ethyl-4-methylimidazole as theimidazole curing accelerator.

The present inventors have now found that a specific combination of theabove-mentioned resin, curing agent and curing accelerator has a quiteunexpected effect as compared with various prior art combinations, andhave completed the present invention. Thus, the epoxy resin compositionsof the present invention are particularly superior to all prior artepoxy resin compositions in the thermal resistance and mechanicalproperties of the cured product. As for thermal resistance, the bendingstrength after heat deterioration of the epoxy resin compositions of thepresent invention is at least four times as high as that of prior artepoxy resin compositions.

2-(3,4-Epoxy)cyclohexyl-5,5-spiro(3,4-epoxy)-cyclohexane-m-dioxane usedin the present invention is represented by the formula ##STR1## and iscommercially available from, for example, Ciba-Geigy Co. under thetrademark of "CY-175".

As the phenol-formaldehyde condensate as a curing agent, already knownones such as, for example, condensates of formaldehyde with phenolcompounds such as 2,2-bis-(4-hydroxyphenyl)propane, pyrogallol,dihydroxydiphenyl, o-, m- and p-hydroxybenzaldehydes, catechol,resorcinol, hydroquinone, phenol, o-, m- and p-chloro- and bromo-phenolsand o-, m- and p-nitrophenols, phenol-xylenedimethoxybenzene condensatesand phenoldimethoxymethyldiphenylether condensates may be used.Particularly useful is phenol-formaldehyde condensate. Theabove-mentioned curing agent is used in an amount of 25 to 50 parts byweight, and preferably 30 to 40 parts by weight, per 100 parts by weightof the above-mentioned epoxy resin (I). If the amount is less than 25parts by weight, thermal resistance becomes poor and reduction inmechanical strength after heat deterioration becomes large. Also, if theamount is larger than 50 parts by weight, cure becomes unsatisfactoryand there is the possibility that electrical properties, andparticularly tan δ-temperature characteristic, deteriorate.

As the curing accelerator, at least one of the above-mentioned imidazolecompounds may be used. In addition to the above-mentioned imidazolecompounds, there are imidazole curing accelerators such as, for example,2-methylimidazole, 2-ethylimidazole, 2-ethyl-4-methylimidazole,1-propyl-2-methylimidazole and 1-benzyl-2-methylimidazole. If theseimidazole compounds are used, however, the storage stability of theresulting epoxy resin compositions (varnishes) or prepreg tape becomesvery poor and the objects of the present invention can not be achieved.

The above-mentioned imidazole compounds used in the present inventionmay be used in an amount of 0.1 to 15 parts by weight, and preferably0.5 to 10 parts by weight, per 100 parts by weight of the epoxy resin.If the amount of the above-mentioned imidazole compounds blended is lessthan 0.1 part by weight, a satisfactory addition effect can not beobtained. Thus, curing temperature becomes higher, curing time isrequired to be prolonged, and the electrical and mechanical propertiesof the cured product are deteriorated. If the amount exceeds 15 parts byweight, the storage stability of the compositions (varnishes) or prepregtape becomes very poor.

According to the present inventors' studies, it has been confirmed thatthe tape winding property of the prepreg tape can be remarkably improvedwithout reducing the heat deterioration resistance by using as thealicyclic epoxy resin a combination of the above-mentioned2-(3,4-epoxy)cyclohexyl-5,5-spiro(3,4-epoxy)cyclohexane-m-dioxane (I)and 3,4-epoxycyclohexylmethyl-3,4-epoxycyclohexanecarboxylaterepresented by the formula ##STR2## Further, the addition of thecompound (II) makes a great contribution toward the control of theviscosity of the varnish. When a combination of the compounds (I) and(II) is used, the amount of the compound (II) is preferably 10 to 60 %by weight per 90 to 40 % by weight of the compound (I), and morepreferably 20 to 50 % by weight per 80 to 50 % by weight of the compound(I). If the amount of the compound (II) blended is less than 10 % byweight, the addition effect can substantially not be obtained. Also, ifthe amount is more than 60 % by weight, heat deterioration resistance inmechanical properties becomes poorer.

Further, the present inventors have confirmed that the thermalresistance (heat distortion temperature) of the cured product can beimproved by using vinylcyclohexenedioxide represented by the formula##STR3## together with the alicyclic epoxy resin (I) or a mixture of thealicyclic epoxy resins (I) and (II). Also, the viscosity control ofvarnishes and the flexibility and winding property around a conductor ofthe prepreg tape can also be improved. When the compound (III) is used,the amount of the compound (III) blended is preferably 5 to 20 parts byweight, and more preferably 8 to 15 parts by weight, per 100 parts byweight of a mixture consisting of 90 to 40 parts by weight of thecompound (I) and 10 to 60 parts by weight of the compound (II). If theamount is less than 5 parts by weight, the addition effect cansubstantially not be obtained. Also, if the amount is more than 20 partsby weight, heat distortion temperature rather disadvantageouslydecreases.

The epoxy resins consisting of the compound (I) and the compound (III)are also effective in the present invention. In this case, the amount ofthe compound (III) blended is preferably 5 to 30 % by weight per 95 to70 % by weight of the compound (I). If the amount blended is less than 5% by weight, improvement in heat distortion temperature can not beexpected. If the amount exceeds 30 % by weight, heat distortiontemperature decreases.

In the present invention, there is no harm in using the compound (II)and/or the compound (III) together with the compound (I) in an amountwhich is less than the above-mentioned lower limits, and there is nopossibility that the properties of the cured product are deteriorated.

When the compound (II) and/or the compound (III) are used together withthe compound (I) in the present invention, the amount of theabove-mentioned curing agent blended may be within the above-mentionedrange per 100 parts by weight of all the epoxy compounds.

The epoxy resin compositions of the present invention are useful forcasting molding, impregnation molding, bonding and coating. Above all,an effective utilization is to use them as a varnish for prepreg. As anepoxy resin prepreg tape or sheet, only those having thermal resistanceof at most class F have heretofore been provided, and silicone resinshave been used in prepreg tapes having thermal resistance of higher thanclass F. However, the silicone resin prepreg tapes have defects in thattheir price is remarkably high, the mechanical strengths of aninsulating layer obtained by winding the tapes around a coil and moldingand then curing the whole are low, and abnormal abrasion of the brush ofrotary electric machines is caused by the generation of SiO₂ on use fora long period of time. The commercial value of the present invention isvery large in that all of the above-mentioned defects can be obviated bythe use of the epoxy resin compositions of the present invention. Whenthe epoxy resin compositions of the present invention are used as avarnish for prepreg, the compositions can be coated onto or impregnatedinto a prepreg substrate as such if the compositions have a lowviscosity. However, it is generally advantageous to use the epoxy resincompositions of the present invention in the form of a solution thereofin an inert organic solvent such as a ketone, a glycol ether, a glycolester, a hydrocarbon or a chlorinated hydrocarbon. As the prepregsubstrate, there may be used papers, fiber woven fabrics, fibernon-woven fabrics, synthetic resin films (tapes), laminated mica sheetand flaky mica sheet. The papers include Japanese paper, linter paper,kraft paper and the other known insulating paper. The fiber wovenfabrics, fiber non-woven fabrics and synthetic resin films (tapes)include inorganic and organic fibrous base materials such as glassfiber, asbestos fiber, cotton, silk, polyester fibers (films), rayonfibers (films), polyamide fibers (films), polyamideimide fibers (films),polyethylene film and polypropylene film. The above-mentioned prepregbase materials are coated or impregnated with the epoxy resincompositions and then dried or semi-cured to obtain the desired epoxyresin prepreg tape or sheet. In this case, the resin concentration ofthe varnish and the amount of the resin deposited on the above-mentionedprepreg substrate are not particularly limited and can be freelyselected.

In the present invention, other epoxy resins may be added to the desiredepoxy resin compositions within such a range as storage stability andthe electrical properties of the cured product are substantially notdeteriorated.

The following examples and reference example illustrate the presentinvention, but the present invention is not limited to these examples.Forexample, the blending ratio of each component can be freely variedwithin the above-mentioned range. Also, use can be selected in the samemanner asin prior art epoxy resin compositions. In the examples andreference example, all parts are expressed by weight unless otherwiseindicated.

EXAMPLE 1

The amounts as shown in Table 1 of2-(3,4-epoxy)cyclohexyl-5,5-spiro(3,4-epoxy)cyclohexane-m-dioxane(CY-175 manufactured by Ciba-Geigy Co.) and a phenol-formaldehydecondensate (HP-203N manufactured by Hitachi Kasei Kogyo K.K., OH value107, softeningpoint 65° - 72° C.) and 1-azine-2-undecylimidazole (C₁₁Z-AZINE) were dissolved in a mixture of 65 parts of methyl ethyl ketoneand 65 parts of acetone to prepare several epoxy resin compositions(varnishes). The gel time (curability) of the above-mentioned varnishesisas shown in Table 1.

The above-mentioned varnishes were then coated onto and impregnated intoa composite substrate obtained by putting a glass cloth of 0.03 mm inthickness on the top of a laminated mica sheet of 0.1 mm in thickness,andthen lightly pressed by a roller. After pre-drying at roomtemperature, thevarnishes were heated at 120° C. for 18 minutes to forma semi-curedstate. Thus, insulating prepreg mica sheets having athickness of 0.20 to 0.26 mm were prepared. Ten sheets of the insulatingprepreg mica sheets were put on the top of one another. The assembly wasplaced on a hot boardof a press at a temperature of about 130° C., and apressure of 30 kg/cm² was applied to the assembly, which was then heatedto 170° C. in 20 to 30 minutes. Curing was carried out by heating andpressing at this temperature for 3 hours to obtain a hard laminatedsheet.The properties of the prepreg sheets and laminated sheets thusobtained areshown in Table 1.

                                      Table 1                                     __________________________________________________________________________                                     Temperature                                                                   at which                                                                             Bending strength                                                       dielectric                                                                           (kg/cm.sup.2) at 180° C                                   Storage                                                                             loss tan-   After                                                       stability                                                                           gent of     deterio-                                             Gel time                                                                             (days) of                                                                           laminated   ration                                          C.sub.11 Z-                                                                        (min.) of                                                                            prepreg                                                                             sheet       at 25° C.                 Sample                                                                            CY-175                                                                             HP-203N                                                                             AZINE                                                                              varnish                                                                              mica  becomes                                                                              Initial                                                                            for                              No. (Parts)                                                                            (Parts)                                                                             (Parts)                                                                            at 150° C.                                                                    sheet 10 % (° C.)                                                                   stage                                                                              30 days                          __________________________________________________________________________    1   100  25    3    10.5   >60   182    1680  861                             2   100  35    3    10.5   40-50 200    2537 1430                             3   100  50    3    12.9   30-40 180    2325 1250                             4   100  30    0.1  6.5    >60   165     815 1490                             5   100  30    5    8.5    40-50 202    2623 1530                             6   100  30    15   5.5    20-30 205    2615 1470                             __________________________________________________________________________

In Table 1, the storage stability of the prepreg sheet was representedas the number of days for which the sheet had to be allowed to standuntil the tape was broken when the sheet was allowed to stand at atemperature of 25° C. and then wound around a 3/16 inch mandrel (40°C.)at 40° C. Also, the dielectric loss tangent was measured at 50 Hz and1000 V. This is also applicable to the other examples as describedbelow.

EXAMPLE 2

The amounts as shown in Table 2 of an epoxy resin mixture consisting of2-(3,4-epoxy)cyclohexyl-5,5-spiro(3,4-epoxy)cyclohexane-m-dioxane(CY-175)and 3,4-epoxycyclohexylmethyl-3,4-epoxycyclohexanecarboxylate(Ch-221 manufactured by Chisso Co.), a phenol-formaldehyde condensate(HP-203N) and 1-azine-2-undecylimidazole (C₁₁ Z-AZINE) were dissolved ina mixture of 65 parts of methyl ethyl ketone and 65 parts of acetone toprepare several epoxy resin compositions. Prepreg mica sheets andlaminated sheets were obtained by the use of the above-mentionedcompositions in the same manner as in Example 1. The properties of thesheets and laminated sheets thus obtained are shown in Table 2.

                                      Table 2                                     __________________________________________________________________________                                        Temperature                                                                   at which                                                                             Bending strength                                                       dielectric                                                                           (kg/cm.sup.2) at 180°                                                  C.                                                               Storage                                                                             loss tan-   After                                                  Gel time                                                                           stability                                                                           gent of     deterio-                                               (min.)                                                                             (days) of                                                                           laminated   ration                                            C.sub.11 Z-                                                                        of var-                                                                            prepreg                                                                             sheet       at 25° C.              Sample                                                                            CY-175                                                                             Ch-221                                                                             HP-203N                                                                             AZINE                                                                              nish at                                                                            mica  becomes                                                                              Initial                                                                            for                           No. (Parts)                                                                            (Parts)                                                                            (Parts)                                                                             (Parts)                                                                            150° C.                                                                     sheet 10 % (° C.)                                                                   Stage                                                                              30 days                       __________________________________________________________________________    7   90   10   35    3    10.7 40-50 198    2381 1497                          8   70   30   35    3    11.0 >60   201    2410 1510                          9   60   40   35    3    11.1 >60   207    2599 1576                          10  40   60   35    3    12.5 >60   183    1962 1101                          __________________________________________________________________________

In Table 2, the storage stability of the prepreg sheet was representedas the number of days for which the sheet had to be allowed to standuntil the tape was broken when the sheet was allowed to stand at atemperature of 25° C. and then wound around a 3/16 inch mandrel (25°C.)at 25° C. This is also applicable to the other examples asdescribedbelow.

EXAMPLE 3

2-(3,4-epoxy)cyclohexyl-5,5-spiro(3,4-epoxy)cyclohexane-m-dioxane(CY-175),vinylcyclohexenedioxide (Ch-206 manufactured by Chisso Co.), aphenol-formaldehyde condensate (HP-203N) and 1-azine-2-undecylimidazole(C₁₁ Z-AZINE) were blended in the ratio as shown in Table 3 toobtainseveral epoxy resin compositions. The gel time of thesecompositions and the heat distortion temperature (HDT) of the curedproducts obtained by heating the compositions at 170° C. for 3 hours areshown in Table 3. Also, the above-mentioned compositions were dissolvedin a mixture of 65 parts of methyl ethyl ketone and 65 parts of acetoneto prepare varnishes. Prepreg mica sheets and laminated sheets were thenprepared by the use of the above-mentioned varnishes in the same manneras in Example 1. The properties of the prepreg mica sheets and laminatedsheets thus obtained are shown in Table 3.

                                      Table 3                                     __________________________________________________________________________                                          Temperature                                                                          Bending strength                                                  Storage                                                                            at which                                                                             (kg/cm.sup.2) at                                         Gel      stabi-                                                                             dielectric                                                                           80° C.                                            time     lity loss tan-   After                                               (min.)   (days)                                                                             gent of     deterio-                                            of       of pre-                                                                            laminated   ration                      Sam-               C.sub.11 Z-                                                                        varnish  preg sheet       at 25° C.            ple                                                                              CY-175                                                                             Ch-206                                                                             HP-203N                                                                             AZINE                                                                              at   HDT mica becomes                                                                              Initial                                                                            for                         No.                                                                              (Parts)                                                                            (Parts)                                                                            (Parts)                                                                             (Parts)                                                                            150° C.                                                                     (° C.)                                                                     sheet                                                                              10 % (° C.)                                                                   stage                                                                              30 days                     __________________________________________________________________________    11 95   5    35    3    9.9  200 40-50                                                                              208    2435 1314                        12 80   20   35    3    9.1  195 >60  200    2388 1286                        13 70   30   35    3    8.7  191 >60  199    2426 1116                        14 70   30   20    3    9.3  183 >60  192    1962 1122                        15 70   30   45    3    11.2 195 30- 40                                                                             197    1976 1304                        __________________________________________________________________________

EXAMPLE 4

2-(3,4-epoxy)cyclohexyl-5,5-spiro(3,4-epoxy)cyclohexane-m-dioxane(CY-175),3,4-epoxycyclohexylmethyl-3,4-epoxycyclohexanecarboxylate(Ch-221), vinylcyclohexenedioxide (Ch-206), a phenol-formaldehydecondensate (HP-203N) and 1-azine-2-undecylimidazole (C₁₁ Z-AZINE) wereblended in the ratio as shown in Table 4 to prepare several epoxy resincompositions. The gel time of these compositions and the heat distortiontemperature (HDT) of the cured products obtained by heating thecompositions at 170° C. for 3 hours are shown in Table 4. Also,theabove-mentioned compositions were respectively dissolved in a mixtureof 65parts of methyl ethyl ketone and 65 parts of acetone to preparevarnishes. Prepreg mica sheets and laminated sheets were then preparedby the use of the above-mentioned varnishes. The properties of theprepreg mica sheets and laminated sheets thus obtained are shown inTable 4.

                                      Table 4                                     __________________________________________________________________________                                                 Temperature                                                                   at which                                                                             Bending strength                                        Gel            dielectric                                                                           (kg/cm.sup.2) at                                                              180° C.                                          time     Storage                                                                             loss tan-   After                                              (min.)   stability                                                                           gent of     deterio-                                           of       (days) of                                                                           laminated   ration               Sam-                     C.sub.11 Z--                                                                       varnish  prepreg                                                                             sheet       at 25°                                                                 C.                   ple CY-175                                                                             Ch-221                                                                             Ch-206                                                                             HP-203N                                                                             AZINE                                                                              at   HDT mica  becomes                                                                              Initial                                                                            for                  No. (Parts)                                                                            (Parts)                                                                            (Parts)                                                                            (Parts)                                                                             (Parts)                                                                            150° C.                                                                     (° C.)                                                                     sheet 10 % (° C.)                                                                   stage                                                                              30                   __________________________________________________________________________                                                             days                 16  90   10   15   35    3    10.2 193 >60   195    2015 1515                 17  70   30   10   35    3    10.0 199 >60   201    2495 1482                 18  70   30   15   35    3    11.3 188 >60   193    2187 1350                 19  60   40   5    35    3    10.7 207 >60   208    2599 1576                 20  60   40   10   35    3    10.5 203 >60   202    2512 1417                 21  60   40   15   35    3    10.1 193 >60   191    2320 1220                 22  40   60   20   35    3    10.8 185 >60   187    1841 1132                 __________________________________________________________________________

EXAMPLE 5

To an epoxy resin system consisting of 55 parts of2-(3,4-epoxy)cyclohexyl-5,5-spiro(3,4-epoxy)cyclohexane-m-dioxane(CY-175), 36 parts of3,4-epoxycyclohexylmethyl-3,4-epoxycyclohexanecarboxylate (Ch-221) and 9parts of vinylcyclohexenedioxide (Ch-206), was added 35 parts of aphenol-formaldehyde condensate (HP-203N, OH value 107, softening point65° - 72° C. or HP-607N, OH value 105, softening point 78° - 86° C.manufactured by Hitachi Kasei Kogyo K.K. or No.1000 WS, softening point99° - 105° C., unreacted phenol content less than 4 % by weightmanufactured by Mitsui Toatsu Kagaku K.K.)as a curing agent, andfurther, as a curing accelerator, 3 parts each of 2-phenylimidazole(2PZ), 2-undecylimidazole (C₁₁ Z), 2-heptadecylimidazole (C₁₇ Z),2-phenyl-4-methylimidazole (2P4MZ), 1-cyanoethyl-2-methylimidazole(2MZ-CN), 1-cyanoethyl-2-ethyl-4-methylimidazole (2E4 MZ-CN),1-cyanoethyl-2-undecylimidazole (C₁₁ Z-CN),1-cyanoethyl-2-phenylimidazole (2PZ-CN), 1-azine-2-methylimidazole(2MZ-AZINE), 1-azine-2-undecylimidazole (C₁₁ Z-AZINE),1-azine-2-ethyl-4-methylimidazole (2E4MZ-AZINE) and1-cyanoethyl-2-methyl-4-ethylimidazole (2M4EZ-CN) were separately added.The mixtures were dissolved in a mixture of 65 parts of methyl ethylketone and 65 parts of acetone to prepare several epoxy resincompositions. Prepreg mica sheets and laminated sheets were prepared bythe use of the above-mentioned compositions in the same manner as inExample 1. The properties of the prepreg mica sheets and laminatedsheets thus obtained are shown in Table 5.

                                      Table 5                                     __________________________________________________________________________                                    Temperature                                                                   at which                                                                             Bending strength                                                       dielectric                                                                           (kg/cm.sup.2) at 180° C.                                  Storage                                                                             loss tan-   After                                                Gel time                                                                             stability                                                                           gent of     deterio-                                             (min.) of                                                                            (days) of                                                                           laminated   ration at                         Sample                                                                            Curing                                                                              Curing   varnish                                                                              prepreg                                                                             sheet become                                                                         Initial                                                                            25° C. for                 No. agent accelerator                                                                            at 150° C.                                                                    mica sheet                                                                          10 % (° C.)                                                                   stage                                                                              30 days                           __________________________________________________________________________    26  HP-203N                                                                             2PZ      8.5    20-30 183    1962 1450                              27  "     C.sub.11 Z                                                                             10.0   40-50 200    2495 1447                              28  "     C.sub.17 Z                                                                             12.5   40-50 185    1971 1482                              29  "     2P4MZ    9.3    20-30 182    1847 1339                              30  "     2MZ--CN  8.5    30-40 186    1852 1427                              31  "     2E4MZ--CN                                                                              9.8    30-40 189    1982 1418                              32  "     C.sub.11 Z--CN                                                                         13.2   >60   199    2521 1462                              33  HP-203N                                                                             2PZ--CN  13.3   >60   185    2027 1425                              34  "     2MZ--AZINE                                                                             8.7    40-50 189    2071 1386                              35  "     2E4MZ--AZINE                                                                           9.0    40-50 192    2035 1395                              36  HP-607N                                                                             C.sub.11 Z--AZINE                                                                      10.2   >60   203    2585 1476                              37  1000WS                                                                              "        9.9    >60   187    2019 1396                              38  HP-203N                                                                             2M4EZ--CN                                                                              9.0    30-40 185    1915 1422                              __________________________________________________________________________

REFERENCE EXAMPLE

2-(3,4-Epoxy)cyclohexyl-5,5-spiro(3,4-epoxy)cyclohexane-m-dioxane(CY-175),3,4-epoxycyclohexylmethyl-3,4-epoxycyclohexanecarboxylate(Ch-221), vinylcyclohexenedioxide (Ch-206), Epikote 152 (manufactured byShell Chemical Co.), a phenol-formaldehyde condensate (HP-203N), borontrifluoride monoethylamine (BF₃ -400 manufactured by Shell Chemical Co.)and 1-azine-2-undecylimidazole (C₁₁ Z-AZINE) were blended in theratio asshown in Table 6. The resulting mixtures were dissolved in a mixture of65 parts of methyl ethyl ketone and 65 parts of acetone to prepareseveral epoxy resin compositions. Prepreg mica sheets and laminatedsheets were prepared by the use of the above-mentioned compositions(varnishes) in the same manner as in Example 1. The curability of theabove-mentioned varnishes and the properties of the prepreg mica sheetsand laminated sheets thus obtained are shown in Table 6.

                                      Table 6                                     __________________________________________________________________________                                                Temperature                                                              Storage                                                                            at which                                                                             Bending strength                                              Gel stabi-                                                                             dielectric                                                                           (kg/cm.sup.2) at 180                                                          ° C.                                                   time                                                                              lity loss tan-    After                                                   (min.)                                                                            (days)                                                                             gent of      deterio-                                                of  of pre-                                                                            laminated    ration               Sam-                    Curing     varnish                                                                           preg sheet        at 25° C      ple                                                                              CY-175                                                                            Ch-221                                                                            Ch-206                                                                            EP-152                                                                            HP-203N                                                                            accelerator                                                                              at  mica becomes                                                                              Initial                                                                             for                  No.                                                                              (Parts)                                                                           (Parts)                                                                           (Parts)                                                                           (Parts)                                                                           (Parts)                                                                            Kind    Parts                                                                            150° C                                                                     sheet                                                                              10 % (° C)                                                                    stage 30                   __________________________________________________________________________                                                             days                 39 55  36  9   --  35   BF.sub.3 400                                                                          3  3.5 >60  81     Impossible                                                                          Impossible                                                              to measure                                                                          to measure           40 55  36  9   --  --   C.sub.11 Z--AZINE                                                                     3  8.7 >60  135    "     "                    41 46  31  23  --  35   "       3  8.8 >60  170    1533  415                  42 --  100 --  --  35   "       3  15.2                                                                              >60  172     521  308                  43 --  --  100 --  35   "       3  13.0                                                                              40-50                                                                              100    Impossible                                                                          Impossible                                                              to measure                                                                          to measure           44 --  --  --  100 35   "       3  3.5 >60  92     "     "                    __________________________________________________________________________

What is claimed is:
 1. An epoxy resin composition comprising analicyclic epoxy resin, a curing agent having phenolic hydroxyl group inits molecule and an imidazole accelerator, characterized in that saidcomposition is obtained by blending 100 parts by weight of2-(3,4-epoxy)cyclohexyl-5,5-spiro(3,4-epoxy)cyclohexane-m-dioxane as thealicyclic epoxy resin, 25 to 50 parts by weight of a phenol-formaldehydecondensate as the curing agent having phenolic hydroxyl group in itsmolecule and 0.1 to 15 parts by weight of at least one imidazolecompound selected from the group consisting of 2-phenylimidazole,2-undecylimidazole, 2-heptadecylimidazole, 2-phenyl-4-methylimidazole,1-cyanoethyl-2-methylimidazole, 1-cyanoethyl-2-ethyl-4-methylimidazole,1-cyanoethyl-2-methyl-4-ethylimidazole, 1-cyanoethyl-2-undecylimidazole,1-cyanoethyl-2-phenylimidazole, 1-azine-2-methylimidazole,1-azine-2-undecylimidazole and 1-azine-2-ethyl-4-methylimidazole as theimidazole curing accelerator.
 2. An epoxy resin composition according toclaim 1, wherein a mixture consisting of 90 to 40% by weight of2-(3,4-epoxy)cyclohexyl-5,5-spiro(3,4-epoxy)cyclohexane-m-dioxane and 10to 60% by weight of3,4-epoxycyclohexylmethyl-3,4-epoxycyclohexanecarboxylate is used as thealicyclic epoxy resin.
 3. An epoxy resin composition according to claim2, wherein a mixture obtained by blending 5 to 20 parts by weight ofvinylcyclohexenedioxide with 100 parts by weight of said mixtureconsisting of2-(3,4-epoxy)cyclohexyl-5,5-spiro(3,4-epoxy)cyclohexane-m-dioxane and3,4-epoxycyclohexylmethyl-3,4-cyclohexanecarboxylate is used as thealicyclic epoxy resin.
 4. An epoxy resin composition according to claim2, wherein the amount of said imidazole curing accelerator contained inthe composition is 0.5 to 10 parts by weight.
 5. An epoxy resincomposition according to claim 1, wherein said composition contains asheet-form substrate.
 6. An epoxy resin composition according to claim2, wherein said composition contains a sheet-form substrate.